The present invention generally relates to an image processing apparatus, and more particularly to an image processing apparatus for processing a multilevel image data of an image by using a binarizing process based on an error scattering method to produce a bi-level processed image suitable for a predetermined printing characteristic of a printing unit that prints the bi-level processed image.
In facsimile machines, a multilevel image data of an image is produced by reading the image from a document, and the multilevel image data is processed by using a binarizing process to produce a bi-level signal. The bi-level signal is stored in a memory, and it is transmitted, received and recorded by a facsimile machine.
Generally, when multilevel image data of an input image is processed by using a binarizing process, a bi-level processed image is produced and a certain amount of image information contained in the input image is discarded after the binarizing process is completed. For this reason, the quality of a processed image which is stored after the binarizing process is completed or received from a transmitting station after it is stored will become lower than quality of the original image.
In order to prevent the quality of a processed image from deteriorating, a known binarizing process based on the error scattering method has been proposed. In the known binarizing process based on the error scattering method, a difference between the input image data density and the processed data density is divided by a given number of neighboring pixels adjacent to a target pixel, and the divided difference is added or scattered in equal proportions to the respective densities of the neighboring pixels. However, when the multilevel image data of an input image is processed by using the binarizing process based on the error scattering method, it is likely that a texture (undesired pattern) appears in the bi-level processed image.
In order to eliminate the undesired pattern from the bi-level processed image, several improved image processing methods have been proposed. For example, Japanese Laid-Open Patent Publication No. 62-239666 discloses an improved image processing method.
In the improved image processing method disclosed in the above mentioned publication, a periodically changing coefficient read from a matrix pattern is added to the multilevel image data relating to one pixel of input image prior to the binarizing process, in order to eliminate the undesired pattern from the bi-level processed image. However, the coefficients of the matrix pattern are fixed and cannot be adjusted. If the manner in which the coefficients of the matrix pattern periodically change does not match the resolution at which the processed image is printed by a printing unit or does not match the type of printing used by a printing unit that prints the processed image, it is likely that the image quality of a printed image will deteriorate even when the improved image processing method is performed.